Telephone test instrument
BACKGROUND OF THE INVENTION 
     This invention relates generally to a telephone test instrument and more particularly, but not by way of limitation, to a telephone technician&#39;s hand-set used in testing subscriber loops of a telephone network. 
     A conventional telephone instrument through which telephone calls are made is referred to as a telephone subscriber set. A subscriber set at a home or business, for example, is connected to a local switching center of the telephone network through wires which extend between the location of the subscriber set and the location of the local switching center. The two wires to which the subscriber set connects are designated tip and ring lines, and the circuit defined from the local switching center to the subscriber set through the two wires is referred to as a subscriber loop. 
     When the subscriber who has the set reports a problem with the telephone circuit, a telephone technician can go to locations along the subscriber loop and connect a portable telephone test instrument, sometimes called a butt-set, across the two wires. Such a test instrument of the type to which we are referring has at least the capability of transmitting voice signals onto and receiving voice signals from the connected loop. typically such an instrument can selectably monitor or seize the circuit, dial into a seized circuit and conduct voice communications. Specific types of such test instruments have various other capabilities. Although such various test instruments exist, there is the need for an improved instrument having other and improved features. 
     SUMMARY OF THE INVENTION 
     The present invention meets the aforementioned need by providing a novel and improved telephone test instrument. The present invention is controlled by a programmed data processor. This permits operations of the instrument to be varied by software changes rather than hardware changes. The present invention uses energy from the telephone circuit, but it also has a rechargeable internal energy supply which provides energy above the level taken from the telephone circuit. The rechargeable energy supply recharges off energy from the telephone circuit. The present invention also has the capability of testing loop current flowing in the subscriber loop without having to disconnect a voice communication circuit of the instrument from the subscriber loop. The present invention also can perform a ground start on the subscriber loop using an external belt hook of the instrument so that an additional connector lead is not needed. An instrument of the present invention also includes a loudspeaker in addition to a receiver and transmitter of the voice communication circuit. During operation of the loudspeaker, the receiver remains active. The loudspeaker is automatically turned off or prevented from being turned on if the voltage of the internal energy supply, which is automatically continually (periodically) checked by the data processor, is too low. An instrument of the present invention includes an overcurrent protection circuit using a positive temperature coefficient resettable fuse and a triac across which the instrument operating circuit of the instrument is connected in parallel. Any particular instrument constructed in accordance with the present invention need not include all of the aforementioned features as indicated by the following combinations defining the present invention. 
     The present invention provides a telephone test instrument, comprising: a housing adapted to be hand-held by a telephone technician; a receiver disposed in the housing; a transmitter disposed in the housing; and circuit means, disposed in the housing and within which the receiver and transmitter are connected, for establishing communication with a telephone circuit, the circuit means including a microprocessor. 
     The present invention provides a telephone test instrument, comprising means for connecting the test instrument to a telephone circuit; a rechargeable energy supply; and means, connected to the means for connecting and to the rechargeable energy supply, for recharging the energy supply from the telephone circuit. 
     The present invention provides a telephone test instrument, comprising: means for connecting the test instrument to a telephone circuit; an internal energy supply; means, connected to the means for connecting, for energizing the instrument from the telephone circuit with a current up to a predetermined current load; and means, connected to the energy supply, for connecting the energy supply to energize the instrument with additional current in response to the instrument requiring current above the predetermined current load. 
     The present invention provides a telephone test instrument, comprising means for connecting the test instrument to a telephone circuit; an internal energy supply; means for measuring a voltage produced by the energy supply; and means, responsive to the means for measuring, for signaling the voltage measurement without an alphabetical or numerical display. 
     The present invention provides a telephone test instrument, comprising: means for connecting the test instrument to a telephone circuit; an internal energy supply; a voice communication circuit operable in either a monitor mode or a talk mode; voice communication circuit switch means for selectably connecting the voice communication circuit to the means for connecting in either the monitor mode or the talk mode; energy supply switch means for selectably connecting the energy supply to provide energy to the instrument; and a data processor connected to the voice communication circuit switch means and the energy supply switch means and programmed for operating the energy supply switch means to connect the energy supply to provide energy to the instrument in response to the voice communication circuit switch means connecting the voice communication circuit to the means for connecting in the talk mode. 
     The present invention provides a telephone test instrument, comprising: a receiver; a transmitter; means for connecting the receiver and the transmitter to a telephone circuit; and means for measuring loop current through the test instrument, the means for measuring connected to the means for connecting so that loop current is measured while the receiver and transmitter are connected to the telephone circuit. This telephone test instrument in a preferred embodiment further comprises means, responsive to the means for measuring loop current, for signaling a loop current measurement without an alphabetical or numerical display. 
     The present invention provides a telephone test instrument, comprising a housing; an electrically conductive hook connected to the housing; means, retained by the housing, for connecting the instrument to a telephone circuit; and a switch mounted on the housing, the switch including two terminals, one of the terminals connected to the hook and the other of the terminals connected to the means for connecting so that the telephone circuit is electrically grounded in response to touching the hook to an electrical ground and closing the switch. 
     The present invention provides a telephone test instrument operable in either a loudspeaker-on mode or a loudspeaker-off mode, comprising: a housing; a receiver disposed in the housing; a loudspeaker disposed in the housing; means for operating the receiver at a base level in response to a signal received by the test instrument over a telephone circuit during the loudspeaker-off mode; means for operating the loudspeaker in response to a signal received by the test instrument over the telephone circuit during the loudspeaker-on mode; and means for operating the receiver at a level not less than the base level during the loudspeaker-on mode. 
     The present invention provides a telephone test instrument, comprising: a receiver; a transmitter; circuit means for connecting the receiver and transmitter to tip and ring lines of a telephone circuit so that loop current of the telephone circuit flows through the instrument, the circuit means including: a positive temperature coefficient resettable fuse; a triac connected to the fuse so that the fuse and the triac are connected in series across the tip and ring lines of the telephone circuit in response to connection of the instrument to the tip and ring lines; and means, connected to the triac, for activating the triac to be conductive in response to a predetermined magnitude of loop current flowing through the instrument so that the loop current flows through the series connected fuse and triac. 
     Therefore, from the foregoing, it is a general object of the present invention to provide a novel and improved telephone test instrument. Other and further objects, features and advantages of the present invention will be readily apparent to those skilled in the art when the following description of the preferred embodiment is read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-1B show one external side view of a telephone test instrument constructed in accordance with the preferred embodiment of the present invention. 
     FIG. 1C shows an enlarged view of a keypad portion shown in FIGS. 1A and 1B. 
     FIG. 2 shows an external side view along line 2--2 shown in FIG. 1. 
     FIG. 3 shows an external side view of the telephone test instrument along line 3--3 shown in FIG. 2. 
     FIG. 4 is an external side view along line 4--4 shown in FIG. 1. 
     FIG. 4A is a view of connectors by which the telephone test instrument can be connected to a telephone circuit. 
     FIGS. 5A-5B are a functional block and schematic diagram of an analog section of the preferred embodiment of the telephone test instrument of the present invention. 
     FIG. 6 is a functional block and schematic diagram of a digital section of the preferred embodiment of the telephone test instrument or the present invention. 
     FlGS. 7A-7D are a schematic circuit diagram of the preferred embodiment of the analog and digital sections of the telephone test instrument of the present invention, excluding at least part of the ringer, ground start and over-current protection circuits. 
     FIG. 8 is a schematic circuit diagram of the preferred embodiment ringer, ground start and over-current protection circuits not shown in FIGS. 7A-7D. 
     FIGS. 9A-9H are flow charts of a program for controlling a microprocessor of the preferred embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment of the telephone test instrument of the present invention is a microprocessor-controlled, high-impact plastic, rugged unit for voice frequency transmission and operational tests of trunk circuits and subscriber lines in a telephone network. With appropriate end clips or plugs, this embodiment can be used by either outside plant or central office personnel. Although the present invention is not limited to a telephone test instrument which includes all of these features or those subsequently described, the features of the preferred embodiment will be described with reference to a single instrument because the presently preferred embodiment includes them. This preferred embodiment, having an external appearance and generally identified by the reference numeral 2 in FIGS. 1-4, includes the following features: 
     An amplifier/speaker with three distinct audio levels provides hands-free listening/monitoring. The speaker is powered by an internal energy supply. If there is sufficient loop current, the energy supply is automatically recharged when the test instrument is connected to a line and the instrument is in the talk mode. During amplifier/speaker operation, an earpiece receiver remains enabled. Amplifier/speaker operation is stopped or prevented if the automatically, continually monitored voltage of the internal energy supply is too low. 
     A microprocessor provides additional features which are controlled by dual-function keys on a keypad. The microprocessor permits operational variations, such as speed of dialing and percentage of break in pulse dialing and frequency in tone dialing, to be made. 
     A repertory dialer will store ten 27-digit numbers with dial modes (tone or pulse). A credit card number and a selectable pause time can be stored along with the number to be called. The last number called can be automatically redialed in either tone or pulse mode. 
     A talk/monitor switch permits talking and monitoring on a connected line. 
     A tone/pulse/out switch selects either DTMF (Dual Tone Multi-Frequency) or DP (dc Dial Pulse) pulsing. A third position of the switch disconnects non-essential circuits to extend the range of the set for listening/talking only. 
     A push-button mute switch allows the user to cut off the transmitter to improve reception in noisy locations The call will remain muted as long as the mute switch is depressed. The switch also prevents the called person from hearing the caller&#39;s private conversation with someone nearby. 
     A tone ringer circuit provides an audible ringing signal when the set is in the monitor mode. 
     A ground start switch provides for originating calls when a ground start condition is required. 
     A spring-loaded belt hook provides a secure connection to a belt loop or &#34;D&#34; ring for a convenient way to carry or store the test set. The belt hook is used as a reference ground path for the optional ground start function. 
     In the monitor mode, line voltage can be measured. 
     In the talk mode, loop current can be measured without changing connections of the test instrument. If the loop current exceeds a predetermined magnitude, a &#34;crow-bar&#34; circuit including a positive temperature coefficient resettable fuse effectively open-circuits to protect the instrument. Over-voltage protection is also provided. 
     Two light-emitting diodes are used to indicate polarity, loop current, line voltage, program version number and internal energy supply voltage. 
     The implementation of these features will become apparent through the following description of the preferred embodiment made with reference to the drawings. External features of the preferred embodiment will be described with reference to FIGS. 1-4. An overview of the electrical analog and digital sections of the preferred embodiment will be given with reference to FIGS. 5 and 6. A description of a particular implementation will be with reference to FIGS. 7-9. 
     External features 
     The instrument 2 has a housing 4 in the shape shown in FIGS. 1-4 The housing 4 is adapted to be hand-held and portable by a telephone technician in the same manner as other types of telephone test instruments known in the art. The housing 4 is preferably made of a relatively lightweight, durable plastic as known in the art. 
     Referring to FIG. 1A, two insulated wires 6, 8 extend from the housing 4. The free end of the wire 6 terminates in a connector 10 (FIG. 4A), and the free end of the wire 8 terminates in a connector 12 (FIG. 4A). The connectors 10, 12 are of suitable types for connecting to the two wires of the telephone circuit to be tested by the instrument 2. Specifically, one connector is used to connect to the tip line and the other connector is used to connect to the ring line. 
     Attached to the housing 4 is an electrically conductive belt hook 14. 
     Also shown in FIG. 1A is a ribbed portion of the housing 4 in which is formed an opening 16 through which a loudspeaker broadcasts when activated. 
     As shown in FIGS. 1A and 1B, a keypad 18 is mounted on the housing 4. Inside the housing 4, the keypad 18 connects to a programmed data processor so that the keypad 18 is the means through which the telephone technician enters data, including external commands, into the processor. 
     Mounted in the housing 4 above the keypad 18 are two light emitting diodes 20, 22. The processor controls the illumination of the light emitting diodes 20, 22 for indicating polarity, loop current, line voltage, program version number and voltage of an internal energy supply. The current and voltage measurements are signaled by serially flashing the diode 20 and then the diode 22 to specify a 2-digit measurement. Thus, the current and voltage measurements are signaled without having to provide space for an alphabetical or numerical display; however, if desired, such a display could be used. 
     Referring to FIG. 1C, the keypad 18 is shown as including 15 keys. As is apparent from the indicia shown in FIG. 1C as the alphanumeric and symbol labeling integrated with by being permanently marked on the keys and housing, eleven of these keys have two functions. Single functions are performed by keys: 
     STORE--store a number in memory 
     RECALL--dial a number previously stored in memory 
     FNCTN--select mode of the dual function key 
     The dual function keys are used for their respective alphanumeric, * or # functions for which they are marked and also for the following functions which are selected after actuating the FNCTN key: 
     1/POLRTY--line polarity test 
     2/REDIAL--redial last number called 
     3/LEVL 3--activate loudspeaker at amplifier level 3 
     4/FLASH--precision hook switch or flash provides 220 millisecond off-hook/on-hook interval 
     5/CURRNT--make loop current measurement in talk mode or loop voltage in monitor mode 
     6/LEVL 2--activate loudspeaker at amplifier level 2 
     7/BATVLT--check internal energy supply voltage 
     8/PAUSE--set a delay interval between series of outpulsed digits (e.g., between a telephone number and credit card number) 
     9/LEVL 1--activate loudspeaker at amplifier level 1 
     0--signal software version number 
     */CHARGE--indicate through light emitting diodes whether in charge mode to recharge internal energy supply 
     #/SPKOFF--turn off the loudspeaker. 
     Referring to FIG. 2, mounted on one side of the housing 4 are two manually activated switches. A talk/monitor switch 24 is a two-position, three-pole rocker switch. A tone/pulse/out (TPO) switch 26 is a three-position, four-section slide switch. 
     As shown in FIG. 3, the housing 4 has an earpiece section 28 and a mouthpiece section 30. Inside the earpiece section 28 is a receiver, and inside the mouthpiece section 30 is a transmitter. Mounted in between these sections in a handle portion of the housing 4 are two push-button, spring-return switches 32, 34. As subsequently described, the switch 32 controls muting of the transmitter, and the switch 34 controls a ground-start circuit. 
     Overview of Analog and Digital Sections 
     Contained within the housing 4 are electrical components and circuits which connect with external components described above and which operate to provide the functions of the preferred embodiment telephone test instrument 2. These components and circuits will be described here in terms of an analog section and a digital section. The analog section is functionally and schematically illustrated in FIGS. 5A-5B, and the digital section is functionally and schematically illustrated in FIG. 6. 
     Analog Section 
     The analog section, and the instrument 2 as a whole, are connected to the telephone circuit through the wires 6, 8 shown in FIGS. 1-4. Referring to FIG. 5A, the wire 6 is connected to or is an extension of a tip conductor 36 of the analog section, and the wire 8 is connected to or is an extension of a ring conductor 38 of the analog section. Connected across the conductors 36, 38 is a ringer circuit 40; and connected to the ring conductor 38 is a ground start circuit 42, which includes the switch 34 also shown in FIG. 3. The circuits 40, 42, and an over-current protection circuit not represented in FIG. 5A, will be more particularly described subsequently with reference to FIG. 8. 
     As indicated by the common labelling in FIGS. 5A and 6, the tip conductor 36 connects into the digital section for permitting polarity and voltage to be tested. The tip conductor 36 also connects through a pole 24a of the talk/monitor switch 24 to either a talk circuit or a monitor circuit via conductors 44, 46 respectively. The talk circuit permits voice communications to be placed on a connected telephone circuit from the transmitter of the instrument 2. Connection to the monitor circuit prevents such transmitted voice communication but permits voice communications to be received from the connected telephone circuit and broadcast through the receiver and loudspeaker of the instrument 2. Such communications are initiated or broadcast through the subsequently described portion of the analog section shown in FIG. 5B. The monitor circuit presents to the telephone circuit a high impedance, such as by a transformer 47 (FIG. 5B). 
     Forming another part of the means for connecting the test instrument 2 to the telephone circuit is a rectifier bridge 48. A node 50 of the bridge 48 connects to the talk circuit conductor 44, and a node 52 of the bridge 48 connects to the ring conductor 38. 
     A node 54 of the bridge 48 is connected to a loop current measurement circuit 56 which is also connected to a section 26a of the TPO switch 26 and into the digital section as indicated by the common labeling in FIGS. 5A and 6. The loop current circuit 56 is operative when the instrument 2 is in both the talk mode (via the switch 24) and either the tone or pulse mode (via the switch 26). When operative, the loop current circuit 56 communicates a signal representative of loop current to a programmed data processor for measurement while the circuit 56 and the receiver and the transmitter of the instrument 2 are connected to the telephone circuit. 
     A node 58 of the bridge 48 connects to the remainder of the circuitry illustrated in FIG. 5A. One of these circuits connects other circuits of the instrument 2 to receive energization from the telephone circuit. This circuit, identified in FIG. 5A by the reference numeral 60, provides up to a predetermined maximum current at a system voltage, V+. Current above the maximum which can be provided through the circuit 60 comes from an internal rechargeable energy supply 62 also disposed in the housing 4. Such current or energy from the energy supply 62 is provided through a circuit 64 which functions as a switch controlled by the data processor of the digital section as indicated by the common labeling of FIGS. 5A and 6. The circuits 60, 64 are connected to a common node 66 through diodes 68, 70. In the particular implementation of the instrument 2 described hereinbelow, the circuits 60, 64 cooperatively operate through the diodes 68, 70 to energize the instrument 2 first from the telephone circuit up to the predetermined maximum current conducted through the circuit 60 and thereafter also from the energy supply 62 connected through the switch circuit 64. 
     Another circuit connected to the node 58 of the bridge 48 is a primary hold current circuit 72 identified in FIG. 5A. The hold circuit 72 is switchable in response to the data processor. When selected to be conductive, the circuit 72 passes a current through a loop current return conductor 73 to hold the telephone circuit to which the instrument 2 is connected. Control of the circuit 72 by the processor is indicated by the common labeling used in FIGS. 5A and 6. 
     Also connected to the node 58 is a circuit for providing a recharging current to the rechargeable energy supply 62 from the telephone circuit. This circuit includes a secondary hold current circuit 74 and a current limiting circuit 76. The circuit 74 is switchable in response to the data processor. When appropriately switched, the circuit 74 passes a current for conduction through the conductor 73 to hold the telephone circuit. In the particular implementation described hereinbelow, the circuit 72 and the circuit 74 operate mutually exclusively of each other so that a hold current is passed only by one of these two circuits 
     The voltage of the energy supply 62 can be measured by the data processor through a voltage monitoring circuit 78. As shown connected in FIG. 5A, the voltage monitoring circuit 78 monitors specifically the voltage of the energy supply 62 only when the switching circuit 64 connects the energy supply 62 to the diode 70. In the preferred embodiment, the data processor automatically continually checks the voltage of the supply 62. 
     Also represented in FIG. 5A is the dialing feature of the instrument 2. This feature permits selectable communication of a telephone number by pulse or by tone in response to the data processor of the digital section. This is accomplished either with a dial pulse signaling circuit implemented through the primary hold current circuit 72, or with a tone signaling circuit 80. The dial pulse signaling circuit is effected by the data processor appropriately modulating the control signal (labeled &#34;PRIMARY HOLD&#34;) which operates the circuit 72. The analog section portion of the tone signaling circuit 80 responds to a tone signal (labeled &#34;DTMF&#34;) received from the digital section to modulate current through the loop current return conductor 73 when the analog section of the tone signaling circuit 80 is enabled through a switching circuit 82 controlled by the data processor, as indicated by the common labeling in FIGS. 5A and 6. In the particular implementation described hereinbelow with reference to FIGS. 7 and 8, the data processor is connected to the dial pulse signaling circuit and to the tone signaling circuit for alternatively controlling the operation thereof so that, for the dial pulse signaling circuit, the speed of transmitting pulses through the dial pulse signaling circuit and the break between the pulses are variable and further so that, for the tone signaling circuit, the generation of one or two tones and the frequency thereof are variable. 
     Referring now to FIG. 5B, the remainder of the analog section will be described. This portion of the analog section includes a receiver 84 which is mounted in the earpiece portion 28 of the housing 4. This portion of the analog circuit also includes a transmitter 86 which is disposed in the mouthpiece portion 30 of the housing 4. This portion of the analog section also includes a loudspeaker 88 disposed in the housing 4 so that the loudspeaker 88 broadcasts through the opening 16 in the portion of the housing 4 shown in FIG. 1A. 
     As illustrated in FIG. 5B, the receiver 84 and the loudspeaker 88 are connected to either the monitor circuit transformer 47 or a conventional hybrid circuit 90 through a pole 24b of the talk/monitor switch 24. The loudspeaker 88 is responsive to the signal received through the pole 24b only if a loudspeaker-driving amplifier 92 is energized. Energization of the amplifier 92 is controlled by the data processor of the digital section through a switching circuit 94 of the analog section shown in FIG. 5B. In the preferred embodiment, the amplifier 92 is de-energized, or not energized, whenever the voltage of the internal energy supply 62 is below a predetermined level as detected during one of the automatic, periodic checks of the voltage made by the data processor. The level of amplification provided by the amplifier 92 is controlled by the data processor through a level or volume control circuit 96 represented in FIG. 5B. The receiver 84 receives either the signal conducted through the switch portion 24b or from the amplifier 92, depending upon the setting of a switching circuit 98 which responds to the setting of the switching circuit 94. 
     The hybrid circuit 90 is a conventional voice communication interface circuit through which the receiver 84, transmitter 86 and loudspeaker 88 (when energized) are connected to the telephone circuit during normal talk mode operation of the instrument 2. The hybrid circuit 90 is selectably connected to the loop current return conductor 73 and an enabling/ disabling switching circuit 100 through TPO switch sections 26b, 26c when the TPO switch 26 is set either in the tone dialing mode or the pulse dialing mode. When the switch 26 is set in the out mode, the switch sections 26b, 26c connect the hybrid circuit 90 directly to the talk circuit conductor 44 and the ring conductor 38. This latter positioning of the switch 26 is typically used only to permit basic voice communication over a telephone circuit which cannot handle the additional load imposed by the remainder of the circuitry of the instrument 2 (e.g., a circuit beyond the subscriber loop limit or a dry circuit). 
     The transmitter 86 is connected to the hybrid circuit 90. The mute switch 32 identified in FIG. 3, for example, has its terminals connected across the transmitter 86. A resistor 102 is connectable across the transmitter 86 in response to closure of a switching circuit 104 controlled by the data processor of the digital section. 
     Digital Section 
     The digital section of the instrument 2 includes the aforementioned programmed data processor which is identified in FIG. 6 by the reference numeral 106. Referring to FIG. 6, the data processor 106 provides the indicated control signals and receives inputs from a pole 24c of the talk/monitor switch 24 and from a section 26d of the TPO switch 26. Further communications are over a bus 108 which connects the processor 106 with a digital-to-analog converter and analog-to-digital converter circuit (DAC/ADC) 110, a dual tone multiple frequency (DTMF) encoder circuit 112 and a random access memory circuit 114. The processor 106 also communicates with the keypad 18, and it controls the light emitting diodes 20, 22. 
     The digital-to-analog converter portion of the circuit 110 provides, under control of the processor 106, a modulating signal for operating the primary hold/dial pulse circuit 72. The analog-to-digital converter portion of the circuit 110 receives the signals by which the processor 106 can measure voltage of the energy supply 62, measure loop current, determine the polarity of the conductors 36, 38 to the tip and ring lines of a telephone circuit to which the instrument 2 is connected, and measure the line voltage across the connected tip and ring lines. The analog-to-digital converter portion of the circuit 110 also receives a signal from the section 26a of the TPO switch 26. 
     The DTMF encoder circuit 112 provides the tone signal to the tone signaling circuit 80 shown in FIG. 5A. The DTMF signal provided by the circuit 112 is generated under control of the processor 106. 
     The random access memory circuit 114 can be used to store a plurality of 27-digit numbers entered into the processor 106 via the keypad 18. Each of these numbers is enough to accommodate a telephone number, pause time and credit card number. 
     Particular Implementation 
     A particular implementation of the circuit more generally depicted in FIGS. 5 and 6 will be described with reference to FIGS. 7-9. Elements shown in FIGS. 7 and 8 corresponding to previously identified elements are referenced with the same numerals (in FIG. 7, the switch 24 is shown in the talk mode position, and the switch 26 is shown in the out mode position). Specific components in FIG. 7 are referred to by their alphanumeric circuit labels. Appendix A contains a list of component values or types correlated to the alphanumeric labels. 
     Referring to FIG. 7A, the particular implementation of the means for connecting the instrument 2 to a telephone circuit is the same as shown in FIG. 5A with the addition of filter capacitor C15, over-current protection device PTC1, and over-voltage protection device MOV1. Connected within this circuit is talk/monitor switch pole 24a which is manually actuated to place the instrument 2 in either the talk mode or the monitor mode. 
     In the talk mode, the transmitter 86 (M1C1) converts sound into electrical signals for transmission through the hybrid circuit 90. This talk circuit is modeled after the Western Electric type 500 telephone set. See also &#34;Lee&#39;s ABC Pocket Guide For The Field On Telephone Set Circuits.&#34; 
     In the monitor mode, the talk circuit is disconnected from the telephone circuit, but the receive circuit is connected across the secondary of the transformer 47 as shown in FIG. 7A. The transformer presents a high impedance (greater than 100K) load to the telephone line for performing non-intrusive monitoring. 
     The data processor 106, particularly implemented by microprocessor U5 (FIG. 7D), determines whether the instrument 2 is operating in the talk mode or the monitor mode by reading the setting of the talk/monitor switch pole 24c. U5 uses P0.1 and P1.7 to read this portion of switch 24. 
     Also shown in FIG. 7A is the analog portion of the loop current measuring circuit 56. This circuit includes resistor R52 having one end connected to the node 54 of the bridge 48 and having its other end connected to section 26a of the TPO switch. In the out position of the TPO switch 26, the resistor R52 is left floating, but in the tone or pulse positions of the switch 26, the resistor R52 is connected to ground so that loop current can flow through R52. 
     To provide a sensing voltage in response to the magnitude of loop current flowing through R52, the loop current measuring circuit 56 includes a Zener diode VR3 having its anode connected to R52 and the node 54 of the bridge 48. VR3 has its cathode connected to resistors R57 and R24. The other end of R57 is connected to the system voltage, V+, and the other end of R24 is connected to capacitor C12 and an analog input of DAC/ADC 110 (U3 in FIG. 7D). This analog input is digitized and communicated to the microprocessor U5 wherein the digitized sensed voltage is used to determine the loop current flowing through R52. For the particular implementation of elements listed in Appendix A, the loop current equals (2.5--voltage read by U5)/R52; however, the &#34;crow-bar&#34; circuit described hereinbelow with reference to FIG. 8 prevents the instrument 2 from conducting over 150 milliamperes of loop current in the particular implementation. 
     When loop current has been measured by the microprocessor U5, its value is signaled without using an alphabetical or numerical display. The loop current value is instead signaled by flashing the light emitting diodes (LEDs) 20, 22 (FIG. 7D). LEDs 20, 22 are controlled by P1.2, P1.6 and P1.7 of U5 through transistors Q20, Q22 and Q21, respectively, shown in FIG. 7D. Q22 is required to disable the LEDs 20, 22 because P1.2 and P1.7 are also used to sense the state of the talk/monitor switch 24 and the TPO switch 26. 
     Referring to FIG. 7B, the analog portion of the power management circuitry of the instrument 2 will be described. When the conductors 36, 38 (FIG. 7A) are connected to an active telephone circuit and the switch 24 is in the talk mode position, energy is provided to the instrument 2 from the telephone circuit through transistors Q7, Q8 (FIG. 7B). In the particular implementation, these transistors provide a 5 milliampere constant current source. Activation of these transistors actuates the microprocessor U5 to perform a processor reset and to set P1.5. The setting of P1.5 activates transistor Q17 through transistor Q23. The switching of these two transistors in turn controls transistors Q16, Q26 so that the internal energy supply 62, which in the particular implementation is a rechargeable nickel cadmium battery, is connected to the node 66 through the diode 70. The transistors Q23, Q17, Q16, Q26 are included within the energy supply switch means 64 schematically represented in FIG. 5A. 
     In the particular implementation, the maximum voltage of the energy supply 62 is less than the voltage at the node 66 when the portions of the instrument 2 being supplied with the system voltage V+ from the node 66 require less than the current which can be provided through the Q7, Q8 energization circuit. When this current is exceeded, the system voltage is pulled down sufficiently so that the diode 70 becomes conductive. Thus, current in excess of that provided from the telephone circuit through Q7, Q8 is provided from the energy supply 62. 
     The switching circuit 64 is activated not only in response to setting P1.5 of the microprocessor U5, but also in response to closure of the FNCTN switch of the keypad 18. Thus, even when the instrument 2 is being operated in the monitor mode by means of the appropriate setting of the switch 24, pressing the FNCTN key activates the circuit 64 to connect the energy supply 62 to provide system voltage V+and thereby actuate the microprocessor U5. This causes the microprocessor to reset itself and then set P1.5 to maintain the energy supply 62 connected through the circuit 64. 
     As previously described with reference to FIG. 5A, the energy supply 62 is rechargeable from the telephone circuit through the secondary hold current circuit 74 and the recharging circuit 76. The secondary or alternate hold circuit 74 includes transistor Q10 which is controlled by transistor Q11 which in turn is controlled through P1.4 of the microprocessor U5. The circuit 74 also includes diode CR13 and resistor R22 connected to Q10 and the loop current return conductor 73. 
     When Q10 is conductive, the recharging circuit 76 is energized to provide a 10 milliampere recharging current through diode CR11 to the energy supply 62. The 10 milliampere current is provided by VR2 in response to the current and voltage from the telephone circuit. 
     In the particular implementation, recharging current is applied to the energy supply 62 only when there is sufficient loop current detected in the telephone circuit via monitoring of the loop current flowing through R52 previously described. Whether recharging current is to be enabled or disabled via control of Q11 is determined by the microprocessor U5 operating within its programmed instructions. This programming is such that no recharging is permitted when the detected loop current falls below a predetermined magnitude, such as 26 milliamperes in the particular implementation. This programming also provides, however, for enabling the recharging circuit in response to the loop current having risen above another predetermined magnitude, such as by increasing to above 30 milliamperes in the particular implementation. This is determined by the microprocessor U5 periodically checking the loop current. In the particular implementation, when not charging but detecting the loop current is above 30 milliamperes, Q11 is actuated to make the circuits 74, 76 conductive. When the loop current falls below 26 milliamperes while charging, however, the microprocessor U5 operates Q11 to disable the circuits 74, 76. These circuits remain disabled until the loop current returns to above 30 milliamperes, whereupon the microprocessor U5 again turns the circuits 74, 76 on. These different current levels are used to provide software-implemented hysteresis to prevent an oscillatory switching on and off condition which could occur if a single on/off current magnitude were used as the switching point. This meets requirements set by BELLCORE to prevent &#34;showering,&#34; a condition wherein telephone equipment is not sure a circuit has been seized. 
     Also shown in FIG. 7B is the primary hold current circuit 72. This circuit provides the primary path for loop current through the talk circuit. The circuit 72 includes transistors Q5, Q6 and Q9. Q6 and Q9 act as a switch and are controlled by Q5. Q5 is controlled by the analog output of the DAC/ADC 110 (U3 in FIG. 7D). Additionally, precision dial pulsing is accomplished through the circuit 72 by modulating the base of Q5 with the analog output of DAC/ADC 110 under control of the microprocessor U5. The software controlling U5 can be changed to vary the dial pulsing, such as its speed and the percentage of break between dial pulses. 
     The instrument 2 can also be operated to provide tone dialing. This includes the DTMF encoder device 112 shown as device U4 in FIG. 7B. Device U4 produces all sixteen valid DTMF tones and eight single frequency tones. The device U4 can also be programmed to output musical tones over a two octave range. U4 operates with an on-chip oscillator running at 3.579545 megahertz and is controlled directly by U5 via the bus 108. 
     To produce DTMF dialing, the talk circuit must be floating to allow U4, through transistor Q12, to drive the loop current return line 73. This is accomplished with transistors Q14, Q15. Q14, when turned on, provides a loop current path for Q12. Q15, when turned on, provides a loop current path for the talk circuit in FIG. 7A. Both Q14 and Q15 are controlled by P1.3 of U5 and are mutually exclusive because of the signal inversion caused by transistor Q13. As indicated in FIG. 7B, Q12, Q14, Q15 correspond to elements 80, 82, 100, respectively, shown in FIGS. 5A and 5B. 
     Microprocessor U5 determines whether the test set is to signal using tone (DTMF) or pulse (dial pulse) by reading section 26d of the TPO switch. Microprocessor U5 uses P0.1 and P1.2 to read this section. Additionally, when the TPO switch 26 is set to the out position, the talk circuit of FIG. 7A is placed directly across the tip and ring lines, bypassing any internal circuitry which would present an extra load on the telephone circuit. Going into out mode also automatically turns off the loudspeaker 88 (FIG. 7C) if it is on. 
     Also shown in FIG. 7B is a voltage divider circuit by which a voltage produced by the energy supply 62 can be measured. This corresponds to the circuit 78 shown in FIG. 5A. Resistors R25, R26 are connected between ground and a monitored voltage, VMON, which tracks the voltage of supply 62 when Q16 is conductive. The junction of R25, R26 is connected to an analog input of the DAC/ADC 110. The sensed analog voltage is digitized and communicated to the microprocessor U5 for conversion to the value of the monitored voltage. This value is communicated via the flashing of the light emitting diodes 20, 22 (e.g., flashing LED 20 to indicate the ones digit and flashing LED 22 to indicate the tenths digit). 
     Referring to FIG. 7C, a voltage reference circuit 116 provides a voltage reference to the DAC/ADC 110 and to the loudspeaker circuit. 
     The loudspeaker circuit shown in FIG. 7C includes the loudspeaker 88, the amplifier circuit 92 and the level/volume setting circuit 96. Also shown in FIG. 7C are particular implementations of the control switch 94 and the switching circuits 98, 104. The switches 98, 104 are parts of a relay K1 shown in FIG. 7C. 
     When the loudspeaker 88 is turned on through control switch 94, K1 is also thereby set. This disconnects the receiver 84 from being driven directly by the talk circuit (i.e., the hybrid 90) and connects the receiver 84 to the output of amplifier device U2 within the circuit 92. This connection is made through R5 and C28 as shown in FIG. 7C. Setting K1 also places resistor R7 (resistor 102 in FIG. 5B) across the transmitter 86. 
     The power for amplifier device U2 and the switching of relay K1 is controlled by transistors Q3, Q4 of the switching circuit 94. Q3 and Q4 are controlled by P0.2 of the microprocessor U5 and transistor Q24. The power to operate U2 and K1 is derived from the internal energy supply 62. Power is applied to U2 and K1 is set when U5, P0.2 is cleared. Additionally, capacitor C1 is charged to provide power to reset K1. Power is removed from U2 and K1 is reset (C1 is discharged through the set reset coils) when P0.2 of U5 is set. 
     During operation of the loudspeaker 88, offset voltage from the circuit 116, through resistors R1, R2, is summed with the received signal to compensate for the unipolar energy supply 62. 
     The received signal is summed with the offset voltage through operational amplifier device U1A, which is configured as a noninverting amplifier with a gain of 2. Additionally, U1A provides a low impedance output to accommodate the input impedance of the circuits 92, 96. 
     The output of U1A is capacitively coupled through capacitor C3 into a digitally controlled voltage divider comprising resistors R9, R10, R11 and transistors Q1, Q2. R9 is the series resistance in the divider. R10 and R11 can be independently switched to ground through Q1 and Q2, respectively. Q1 and Q2 are controlled by P1.0 and P1.1, respectively, of the microprocessor U5. The output of the digitally-controlled voltage divider provides input to amplifier device U2, which is configured for maximum open loop gain. Three of the four discrete levels of gain which can be obtained through U2 depending upon the states of Q1 and Q2 as controlled by the microprocessor U5 are selected in response to actuation of appropriate keys of the keypad 18 and the state of the talk/monitor switch 24 as will be subsequently described. 
     To summarize, actuating the control switch 94 energizes the amplifier circuit 92 and operates the switch 98 (and the switch 104) so that the receiver 84 is driven in response to the output signal from the amplifier circuit 92. When the loudspeaker 88 is not energized, the receiver 84 is operated at a base level from the hybrid circuit 90. During loudspeaker operation when the switch 98 connects the receiver 84 to the amplifier circuit 92, the receiver 84 remains active and is operated at a level not less than the base level. Thus, the receiver 84 remains active at all times in both loudspeaker-on and loudspeaker-off modes. In the loudspeaker-on mode, the amplifier circuit 92 also drives the loudspeaker 88. The particular level at which the receiver 84 and the loudspeaker 88 operate is set by the circuit 96. During the loudspeaker-on mode, the resistor 102 (R7) is connected across the transmitter 86. 
     Referring to FIG. 7D, the programmed data processor 106 is shown implemented by the microprocessor U5. The particular implementation of U5 is known. The interface between U5 and the connected keypad 18 is described in Appendix B hereof. Also connected to U5 are an oscillator circuit 118 and a reset circuit 120. The program contained within the internal program memory of U5 is listed in Appendix C hereof. Flow charts of the program are shown in FIGS. 9A-9H. 
     FIG. 9A shows the flow chart for the reset or system initialization portion of the program. Reset is caused by any one of the following: pressing the function (FNCTN) key of the keypad 18, switching from monitor mode to talk mode when the instrument 2 is connected to active tip and ring lines, switching from tone mode or pulse mode to out mode, switching from out mode to tone mode or pulse mode, and in the SYS/BAT routine if the internal energy supply 62 voltage is not greater than a predetermined voltage. When the reset routine is entered, the voltage of the supply 62 is checked. If it is below the predetermined value, and the instrument 2 is in the talk mode and there is sufficient loop current, the battery is charged. If the voltage of the supply 62 is initially above the predetermined voltage, the out mode and monitor mode are checked and the appropriate action shown in FIG. 9A taken. 
     FIG. 9B shows the monitor routine which can be entered from the reset routine as indicated in FIG. 9A. In the monitor routine, the functions indicated in FIG. 9B can be selected by means of pressing the appropriate keys of the keypad 18. It is to be noted that in the monitor routine, the internal energy supply voltage is checked when loudspeaker operation is selected. U5 continually (22.7 seconds) and automatically checks the voltage to insure that it is above a predetermined level adequate for energizing the loudspeaker. This also occurs in the SYS/BAT routine (FIG. 9E) and function routine (FIG. 9H). 
     FIG. 9C shows the talk routine which can also be entered from the reset routine. The talk routine includes performing the loop current routine, the SYS/BAT routine and the keypad input routine, each of which will be described hereinbelow with reference to FIGS. 9E, 9F and 9G, respectively. 
     FIG. 9D is the flow chart for the talk mode/off-line routine which can be entered from the loop current routine shown in FIG. 9E. During the talk mode/off-line routine, telephone numbers with pauses can be stored. 
     FIG. 9E shows the loop current routine which controls the charging of the internal energy supply 62. This routine implements software charge hysteresis wherein if the supply is not being charged and the loop current is greater than a first predetermined level, U5 turns on the charging circuit and turns off the primary hold circuit; whereas if the charge circuit is on and the loop current is less than a second predetermined value, U5 turns off the charging circuit and turns on the primary hold circuit. FIG. 9E also shows that if the loop current is initially detected to be below a minimum value, the instrument 2 is considered to be off-line so that the talk mode/off-line routine is entered. 
     FIG. 9F is the flow chart for the SYS/BAT routine. If the internal energy supply 62 voltage is too low, the reset routine is entered. If the supply voltage is initially above the predetermined level, it is determined whether the loudspeaker is on. If it is not, modes set by the talk/monitor switch 24 and the TPO switch 26 are checked. If, on the other hand, the speaker is on, the internal energy supply 62, is checked, as is a five-minute operating limit. Ultimately the indicated modes are checked or the system is turned off as indicated. 
     FIG. 9G shows the keypad input routine wherein the talk routine is recommenced, or the function routine is entered, or store, recall and dialing functions are checked and performed. During a store function, a telephone number is loaded in a selected memory location; if a recall function is performed, a previously entered number is retrieved and dialed from the selected location; in the dialing function, the telephone number is dialed by either DTMF or pulse dialing and the number is stored in the last number redial location. 
     FIG. 9H shows the function routine which is entered from the keypad input routine shown in FIG. 9G. The various functions are performed if the function (FNCTN) and appropriate numerical/function selection keys of the keypad 18 are pressed. These various functions are described hereinbelow in the &#34;operation&#34; section. 
     With reference again to FIG. 7D, also shown in FIG. 7D are switch sections 24c, 26d which are read by U5 to determine the talk/monitor and tone/pulse/out modes of operation of the instrument 2. 
     The LEDs 20, 22 which are controlled by U5 to indicate polarity, loop current and internal energy supply voltage are also shown in FIG. 7D. To conserve energy, U5 duty-cycles or strobes the LEDs 20, 22 when they are illuminated. 
     Also shown in FIG. 7D is the DAC/ADC 110 implemented by integrated circuit U3. U3 provides one 8-bit digital-to-analog converter (DAC) and four 8-bit analog-to-digital converters (ADC) in the single integrated chip. All channels are controlled directly from U5 via the bus 108. U3 derives a reference for the ADC from the circuit 116 shown in FIG. 7C. The DAC controls the base of Q5 which controls the hold/dial pulse circuit 72. 
     ADC channel 1 receives the voltage which is on the tip conductor 36 to determine whether the polarity of the test leads 6, 8 is correct. This input is scaled by resistors R27, R28 shown in FIG. 7C. This scaling also allows the line voltage to be measured in the monitor mode when the polarity is correct. 
     ADC channel 2 senses the out position of the TPO switch 26. 
     ADC channel 3 receives the voltage of the voltage monitor circuit 78 comprising resistors R25, R26 connected to Q16 (FIG. 7B). 
     ADC channel 4 receives voltage responsive to the current through the resistor R52 to determine loop current. 
     Also shown in FIG. 7D is the random access memory 114 implemented by the integrated circuit U6. U6 provides 256 bytes of random access memory for the storage of phone numbers. U6 is powered directly from the internal energy supply 62 so that it will retain its contents even when the instrument 2 is off line. U6 is controlled directly by U5 via the bus 108. 
     Referring to FIG. 8, implementations for the ringer circuit 40 and the ground start circuit 42 are shown. The circuit 40 includes a piezo ringer 122 which is activated by the illustrated circuit which is connected to the ring and tip junctions having the common labeling (R and T) in FIGS. 7A and 8. The ringer 122 produces a ring tone when the test instrument 2 is rung into while in the monitor mode. 
     The ground start circuit 42 includes a resistor 124 connected between the ring conductor 38 (via the R junction) and the switch 34 (via the GS1 junction). The other end of the switch 34 is connected through the circuit 42 to the belt hook 14 (via the GS2 and GND junctions). When the switch 34 is closed and the belt hook 14 is touched to an electrical ground, the ring conductor of the connected telephone circuit is grounded through the resistor 124. 
     Also shown in FIG. 8 is part of the high current protection circuit of the illustrated embodiment. The portion of the circuit shown in FIG. 8 includes a resistor 126 connected to an opto-coupled triac 128 which is also connected to resistors 130, 132 and a 20-ampere triac 134. This circuit is connected to the R, T, TT, A and BRG- junctions as shown in FIGS. 8 and 7A. This places the triac 134 in series with the positive temperature coefficient resettable fuse, PTC1, between the tip and ring lines when the instrument 2 is connected to the tip and ring lines of a telephone circuit. The triac 134 is also in parallel with the instrument operating circuit which includes the bridge 48 and the circuits connected off the nodes 54, 58 thereof. 
     When the instrument 2 is connected to the tip and ring lines and operated in the talk mode, the loop current to the instrument 2 is routed through R52 (FIG. 7A). This allows loop current to be calculated by measuring the voltage drop across R52. Additionally, R52 is connected across the input of the optocoupled triac 128 through the A and BRG- junctions (FIGS. 7A and 8). The output of the opto-coupled triac 128 operates the gate of the triac 134. 
     If the instrument 2 is exposed to a constant high voltage source such as a power line (AC) or pair-gain equipment (DC), large amounts of current will flow through CR12 (FIG. 7A) or, if in charge mode, CR17 (FIG. 7B). When the current exceeds 150 milliamperes in the particular implementation, the voltage drop across R52 will bias on the internal light emitting diode of the opto-coupled triac 128, thereby triggering its internal triac and gating on the triac 134. When this occurs, the triac 134 presents a virtual short, shunting the loop current through itself and PTC1. 
     PTC1 is a positive temperature coefficient resistance device which exhibits a non-linear increase in resistance in response to increased current. In the particular implementation, PTC1 presents a series resistance of 8.5 ohms until the predetermined current of 150 milliamperes is reached. At that time, the resistance of PTC1 increases by several orders of magnitude, thereby presenting a virtual open circuit in response to the high loop current passing through it. This protects the instrument 2 until the hazardous current is removed. Removing the external voltage from the instrument 2 of the particular implementation is the only way this &#34;crow-bar&#34; circuit is reset. 
     With reference again to diodes CR12, CR17 mentioned above, these two elements provide high voltage surge protection along with MOV1 (FIG. 7A). In the event of a high voltage surge, such as by lightning, MOV1 will clamp the voltage to approximately 300 volts in the particular implementation providing over-voltage protection for PTC1. Due to the response time of MOV1, however, a momentary voltage surge could be transmitted through the bridge 48. Such a voltage surge will be clamped to 144 volts in the particular implementation by CR12. If a voltage surge were to occur while the instrument 2 is in the charge mode (using the current path through Q10 as the hold circuit), CR17 (FIG. 7B) will clamp the voltage to 40 volts in the particular implementation, thereby protecting VR2, which has a maximum voltage rating of 40 volts in the particular implementation. 
     Operation 
     internal energy supply check 
     Before the instrument 2 is used, the condition of the internal energy supply 62 is checked by pressing the FNCTN and 7/BATVLT keys. The range of operation for the loudspeaker/amplifier in the particular embodiment is from 4.6 to 5.9 volts. The other functions of the instrument 2 will operate on a low limit of 2 volts. The LED 20 flashes at 30 ipm (interruptions per minute) indicating the units digit; the LED 22 flashes at 30 ipm indicating the tenths digit. A low limit reading would be 4 flashes of the LED 20 and 6 flashes of the LED 22. 
     connecting to a line 
     Operate the talk/monitor switch 24 to the monitor position before connecting to a subscriber line to avoid interrupting any transmission in progress. The high impedance monitoring circuit provides a non-service affecting audible monitor. The amplifier/loudspeaker can be enabled with similar results. 
     Connect the lead 6 of the test instrument 2 to the tip (positive/ground) side of the telephone circuit and the lead 8 to the ring (negative/battery) side of the telephone circuit. If the leads are connected in reverse, the instrument 2 will work; however, a polarity check will show a reverse polarity. 
     enabling the amplifier speaker 
     Press the FNCTN key plus one of the LEVEL keys to enable the amplifier/speaker at one of four amplification levels: AL0 (minimum), AL1, AL2, AL3 (maximum). 
     
         ______________________________________                                    
               monitor mode                                               
                        talk mode                                         
______________________________________                                    
FNCTN + 9/LEVL 1 AL1        AL0                                           
FNCTN + 6/LEVL 2 AL2        AL1                                           
FNCTN + 3/LEVL 3 AL3        AL2                                           
______________________________________                                    
 
    
     The amplifier will be automatically powered down after 5 minutes or if the internal energy supply 62 voltage drops too low. 
     amplifier/speaker off 
     Press the FNCTN and #/SPKOFF keys to turn off the amplifier/loudspeaker. 
     loop current/line voltage check 
     To make a check of the loop current, operate the talk/monitor switch 24 to the talk position and press the FNCTN and 5/CURRNT keys. The number of flashes of the LED 20 will indicate the tens digit of the measured current (in milliamperes); the units digit will be indicated by counting the number of flashes of the LED 22. The measurement range of the particular illustrated embodiment is from 0-250 milliamperes with regards to the loop current measurement circuit 56 itself, but the overall circuitry is limited to 150 milliamperes by the &#34;crow-bar&#34; circuit of FIG. 8. That is, if loop current exceeds 150 milliamperes, PTC1 effectively open-circuits. 
     If the FNCTN and the 5/CURRNT keys are pressed in the monitor mode, the voltage sensed at the junction of R27, R28 will be measured and displayed via flashing of the LEDs 20, 22. If the instrument is connected in reversed polarity, the voltage will not be displayed; only one LED will be flashed to indicate &#34;low&#34;. 
     charge current check 
     Although when in the talk mode pressing the FNCTN and 5/CURRNT keys produces signals indicating the loop current magnitude, pressing the FNCTN and */CHARGE keys in the talk mode illuminates one of the LEDs 20, 22 to indicate the charged status of the energy supply 62. LED 20 is flashed at two-second intervals if the voltage of the supply 62 is less than 5.8 volts; LED 22 is flashed at two-second intervals if the voltage is greater than or equal to 5.8 volts. This function also puts the instrument in the charge mode if charging current is available. 
     polarity check 
     If a polarity check on the line is desired, operate the talk/monitor switch 24 to the monitor position and press the FNCTN and the 1/POLRTY keys. A lighted LED 22 indicates normal polarity; a reversed line will be indicated by a lighted LED 20. 
     originating a call 
     With the instrument 2 connected to an idle line, select the type of pulsing required by moving the TPO slide switch 26 to position T if DTMF pulses are required or to position P for dc pulses. 
     Set the talk/monitor switch 24 in the talk position and verify that dial tone is received by listening to the receiver 84. With the keypad 18, enter the desired number. The digits will be output at the correct rate. 
     To draw dial tone on a ground start line, operate the talk/monitor switch 24 to the talk position and press the ground start switch 34 while holding the belt hook 14 against a grounded object. 
     To mute the transmitter, such as may be needed in noisy locations, press and hold the mute switch 32. 
     last number redial 
     In T (DTMF) mode or P (dc pulse) mode, the last number dialed can be automatically redialed by toggling the talk/monitor switch 24 from talk position to monitor position to talk position and then pressing the FNCTN key followed by the 2/REDIAL key. 
     storing numbers in the repertory dialer memory 
     The particular implementation of the instrument 2 has 10 memory locations allocated for storing telephone numbers; one for each of the number keys 1 through 0. Twenty-seven digits can be stored in each of the locations; if a 28th digit is entered, the previous digits will be cleared and the last digit (the 28th just entered) starts a new series. Numbers can be entered in memory at any time. That is, the test set may be either on-hook or off-hook and need not be connected to a subscriber loop. 
     In some situations, it may be necessary to insert a pause between digits of a stored number, e.g., when accessing a trunk through a PBX or CENTREX switch that requires a &#34;9&#34; or other intermediate access number to seize an outside line. This is accomplished by pressing the FNCTN key and the 8/PAUSE key. Each time the FNCTN and 8/PAUSE keys are operated in this manner, a 4-second delay will be stored in the number string. A maximum of four 4-second pauses may be stored between any two numbers in a string. 
     A credit-card number can be stored along with the number being called. Pause intervals from 4 to 16 seconds are programmable and use only one digit of the register. A call requiring credit-card billing will be stored as follows: 0+3 digit area code+7 digit telephone number+8 second pause+14 digit credit-card number, e.g., 0+123+456-7890+(FNCTN+8/PAUSE twice)+405+555-1212-2934. 
     To store a number being called: 
     (1) connect the test instrument 2 to a subscriber line; 
     (2) if the line is idle, operate the talk/monitor switch 24 to the talk position; 
     (3) dial the number; 
     (4) press the STORE key; 
     (5) press one of the keys 1 through 0 to designate the memory location. 
     A call will be placed to the number dialed; the memory location will be cleared if a number was previously stored there, and the new number will be stored in that location. 
     To store a number with the set on hook or disconnected: 
     (1) select talk mode via switch 24; 
     (2) press the FNCTN key once; 
     (3) enter the number to be stored; 
     (4) press the STORE key; 
     (5) press a number key, 1 through 0, for the desired memory location. (If there is no keypad activity for 15 seconds, the number will be lost and the procedure must be repeated.) 
     dialing a stored number 
     After going off-hook and receiving dial tone, press the RECALL key and then the appropriate key (0-9) for the desired memory location. The number selected will be automatically dialed in the dial mode selected by the tone/pulse/out switch 26. 
     precision hookswitch flash 
     Press the FNCTN and 4/FLASH keys when a precision-timed interval (flash) is required to eliminate accidentally disconnecting calls. This provides an electronically-precision-timed disconnect for custom calling and PABX features, e.g., while testing trunk circuit equipment. 
     software version number 
     Pressing the FNCTN and 0 keys causes the software version number to be signaled by flashing the LEDs 20, 22 to count the version number. 
     disconnection upon test or call completion 
     After testing is completed, release the line or equipment by operating the talk/monitor switch 24 to the monitor position or by disconnecting the test clips from the line or by removing the plug from the equipment. 
     Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While a preferred embodiment of the invention has been described for the purpose of this disclosure, changes in the construction and arrangement of parts can be made by those skilled in the art, which changes are encompassed within the spirit of this invention as defined by the appended claims. 
     APPENDIX A 
     U1 dual operational amplifier integrated circuit, LT1013CN8 
     U2 power amplifier integrated circuit, LM386N 
     U3 I 2  C bus D/A×1, A/D×4 integrated circuit, Signetics PCF 8591 
     U4 I 2  C bus DTMF generator integrated circuit, Signetics PCD 3312 
     U5 I 2  C bus 2K EPROM microprocessor integrated circuit, Signetics PCF83C751 
     U6 I 2  C bus 256K×8 static RAM integrated circuit, Signetics PCF8570 
     R1 1M (resistor values are in ohms) 
     R2 10K 
     R3 20K 
     R4 16K 
     R5 6.8K 
     R6 43K 
     R7 5.1 
     R8 160K 
     R9 1K 
     R10 470 
     R11 110 
     R12 11K 
     R13 510K 
     R14 33K 
     R15 10K 
     R16 130 
     R17 160K 
     R18 120 
     R19 27 
     R20 10K 
     R21 100K 
     R22 560, 3W 
     R24 10K 
     R25 69.8K 
     R26 100K 
     R27 25.5K 
     R28 1M 
     R29 100K 
     R30 10K 
     R31 180 
     R32 30K 
     R33 75K 
     R34 1M 
     R35 100 
     R36 2K 
     R37 2K 
     R38 120 
     R39 39K 
     R40 3.3K 
     R41 39 
     R42 100K 
     R43 100K 
     R44 3.3K 
     R45 5.1 
     R46 100 
     R47 470 
     R48 82 
     R50 100K 
     R51 100K 
     R52 10, 1%, 1.5W 
     R53 100K 
     R54 8.2K 
     R55 1M 
     R56 100K 
     R57 43K 
     C1 220 (capacitor values are in microfarods unless otherwise marked) 
     C2 0.47 
     C3 2.2 
     C4 0.1 
     C5 0.1 
     C6 10 
     C7 0.1 
     C9 220 
     C10 0.1 
     C11 0.1 
     C12 1.0 
     C13 0.47 
     C14 4.7 
     C15 0.01 
     C16 2.2nf 
     C17 0.1 
     C18 10 
     C19 22pf 
     C20 22pf 
     C21 10 
     C22 0.47 
     C23 1.5 
     C24 4.7 
     C25 0.1 
     C27 10 
     C28 0.1 
     C29 0.01 
     C31 0.1 
     VR1 LT1004-2.5 
     VR2 LT1086 
     VR3 LT1004-2.5 
     CR12 130V transient voltage suppressor 
     CR17 37V transient voltage suppressor 
     MOV1 300V metal-oxide varistor 
     PTC1 positive temperature coefficient resettable fuse 
     MIC1 Audiosears 1583 
     REC1 Audiosears 2463V 
     SPK1 Star Micronics DS-29C, 45 ohms, 0.1W 
     APPENDIX B 
     Keypad Scanning 
     I/O port 3 (P3.0-P3.7) of U5 is configured to scan the keypad. P3.4-7 are programmed as outputs and sequence a &#34;0&#34; through the rows of the keypad. P3.0.3 are programmed as inputs and read the columns. The diagrams and table detail the keypad encoding scheme, the configuration of P3 and the bit mapping and corresponding DTMF output codes. ##STR1## 
     
                       KEYPAD LOOK-UP TABLE                                        
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DTMF CODE      OUTPUT       INPUT                                         
KEY   BINARY    HEX    BINARY  HEX  BINARY  HEX                           
______________________________________                                    
0     XX010000  10     0111XXXX                                           
                               7X   XXXX1011                              
                                            XB                            
1     XX010001  11     1101XXXX                                           
                               DX   XXXX0111                              
                                            X7                            
2     XX010010  12     1101XXXX                                           
                               DX   XXXX1011                              
                                            XB                            
3     XX010011  13     1101XXXX                                           
                               DX   XXXX1101                              
                                            XD                            
4     XX010100  14     1110XXXX                                           
                               EX   XXXX0111                              
                                            X7                            
5     XX010101  15     1110XXXX                                           
                               EX   XXXX1011                              
                                            XB                            
6     XX010110  16     1110XXXX                                           
                               EX   XXXX1101                              
                                            XD                            
7     XX010111  17     1011XXXX                                           
                               BX   XXXX0111                              
                                            X7                            
8     XX011000  18     1011XXXX                                           
                               BX   XXXX1011                              
                                            XB                            
9     XX011001  19     1011XXXX                                           
                               BX   XXXX1101                              
                                            XD                            
? (A) XX011010  1A     1101XXXX                                           
                               DX   XXXX1110                              
                                            XE                            
R (B) XX011100  1B     1110XXXX                                           
                               EX   XXXX1110                              
                                            XE                            
FNC   XX011101  1C     1011XXXX                                           
                               BX   XXXX1110                              
                                            XE                            
(C)                                                                       
S (D) XX011011  1D     0111XXXX                                           
                               7X   XXXX1110                              
                                            XE                            
*     XX011110  1E     0111XXXX                                           
                               7X   XXXX0111                              
                                            X7                            
#     XX011111  1F     0111XXXX                                           
                               7X   XXXX1101                              
                                            XB                            
______________________________________                                    
 ##SPC1##